I had a lumbar puncture about 18 mos ago to determine if I have psuedotumor cerebri AKA intercranial hypertension. In my case I was given a mild sedative, the spot was numbed and I laid on my stomach on the table while the procedure was done. It was not painful, but the insert of the needle into the canal and then withdrawal of fluid was somewhat uncomfortable. I was able to watch most of the procedure on the x-ray as it was being done and because the sedative was mild, could ask questions about it. I'd always heard horror stories about having a "spinal tap", but this really was no big deal. Don't know how common my experience is, but hopefully yours is similar!

Best wishes and hope you get to feeling better!